/*
 * Licensed to the Apache Software Foundation (ASF) under one
 * or more contributor license agreements.  See the NOTICE file
 * distributed with this work for additional information
 * regarding copyright ownership.  The ASF licenses this file
 * to you under the Apache License, Version 2.0 (the
 * "License"); you may not use this file except in compliance
 * with the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.apache.giraph.combiner;

import org.apache.hadoop.io.Writable;
import org.apache.hadoop.io.WritableComparable;

/**
 * Abstract class to extend for combining messages sent to the same vertex.
 * Combiner for applications where each two messages for one vertex can be
 * combined into one.
 *
 * @param <I> Vertex id
 * @param <M> Message data
 */
public abstract class Combiner<I extends WritableComparable,
    M extends Writable> {
  /**
   * Combine messageToCombine with originalMassage,
   * by modifying originalMessage.
   *
   * @param vertexIndex Index of the vertex getting these messages
   * @param originalMessage The first message which we want to combine;
   *                        put the result of combining in this message
   * @param messageToCombine The second message which we want to combine
   */
  public abstract void combine(I vertexIndex, M originalMessage,
      M messageToCombine);

  /**
   * Get the initial message. When combined with any other message M,
   * the result should be M.
   *
   * @return Initial message
   */
  public abstract M createInitialMessage();
}
